U1 RNA is an abundant low molecular weight nuclear RNA species. Some patients with autoimmune disorders produce antibodies that recognize a ribonucleoprotein particle, named RNP, whose constituents are U1 RNA plus a few proteins (Proc. Nat. Acad. Sci. USA 76, 5495 (1979). U1 RNA may be involved in the splicing of messenger RNA, in view of the nucleotide sequence complementary between the 5' end region of U1 RNA and the consensus sequence at the splice junctions of eukaryotic messenger RNAs (Nature 283, 220 (1980); Proc. Nat. Acad. Sci. USA 77, 1877 (1980). In addition, the anti-RNP antibodies inhibit the in vitro splicing of adenovirus messenger RNA (Proc. Nat. Acad. Sci. USA 78, 1371 (1981)). We are working on the conditions necessary to obtain in vitro splicing of adenovirus messenger RNA in a soluble system. This system would then be used to test the activity of the RNP particle and its components. Another goal is to characterize the proteins of the RNP particle and of a similar ribonucleoprotein complex. We also wish to know if more copies of these proteins are present in the nucleus, not associated with RNP or Sm. Another objective is to determine if U1 and U2 RNA, in their apparent passage through the cytoplasm shortly after their synthesis, are already incorporated into cytoplasmic particles recognized by anti-Sm or anti-RNP antibodies.